The planned experiments develop two general themes: (1) systematic changes in the properties of circadian systems as a function of age in insects and mammals; (2) a dependence of physiological performance on the proximity of a circadian system to "resonance". Under (1) the main concern is with age-dependent changes in the period and homeostatic stability of the pacemakers driving activity and body temperature cycles. We are also exploring the lability of circadian organization in very young animals. Under (2) the main concern is with an impairment of growth rates and longevity attributable to light cycles whose period length is different from 24 hours.